diff --git a/parser.c b/parser.c
index eed646b..fc40274 100644
--- a/parser.c
+++ b/parser.c
@@ -165,11 +165,18 @@ static bool skip(char ch)
 	return in_comment;
 }
 
+/* Used for parsing octal numbers */
+static int isodigit(int c)
+{
+	return isdigit(c) && c < '9';
+}
+
 static struct token yylex()
 {
 	struct symbol *sym;
 	struct token token = {};
 	unsigned long long num;
+	int (*isvalid)(int);
 	char buf[128];
 	char *p = buf;
 	int base;
@@ -214,20 +221,31 @@ static struct token yylex()
 
 		return token;
 	} else if (isdigit(ch)) {
+		/* Determine base and valid character set */
+		if (ch == '0') {
+			*p++ = ch;
+			ch = input();
+			if (ch == 'x' || ch == 'X') {
+				*p++ = ch;
+				ch = input();
+				isvalid = isxdigit;
+				base = 16;
+			} else {
+				isvalid = isodigit;
+				base = 8;
+			}
+		} else {
+			isvalid = isdigit;
+			base = 10;
+		}
+
 		do {
 			*p++ = ch;
 			ch = input();
-		} while (isxdigit(ch) || (p - buf == 1 && ch == 'x'));
+		} while (isvalid(ch));
 		unput(ch);
 		*p = '\0';
 
-		if (buf[0] == '0' && buf[1] == 'x')
-			base = 16;
-		else if (buf[0] == '0')
-			base = 8;
-		else
-			base = 10;
-
 		errno = 0;
 		num = strtoull(buf, NULL, base);
 		if (errno)
diff --git a/tests/bad_X.qmi b/tests/bad_X.qmi
new file mode 100644
index 0000000..d648835
--- /dev/null
+++ b/tests/bad_X.qmi
@@ -0,0 +1,19 @@
+package test;
+
+struct qmi_result {
+	u16 result;
+	u16 error;
+};
+
+request test_request {
+	# Previously "0X" was not allowed to indicate a hexadecimal value
+	required u8 test_number = 0X12;
+} = 0x23;
+
+response test_response {
+	required qmi_result r = 2;
+} = 043;
+
+indication test_indication {
+	optional u64 value = 0x99;
+} = 0x7;
diff --git a/tests/bad_decimal.qmi b/tests/bad_decimal.qmi
new file mode 100644
index 0000000..b9f8f2e
--- /dev/null
+++ b/tests/bad_decimal.qmi
@@ -0,0 +1,19 @@
+package test;
+
+struct qmi_result {
+	u16 result;
+	u16 error;
+};
+
+request test_request {
+	required u8 test_number = 0x12;
+} = 0x23;
+
+response test_response {
+	# Note that 'a' is not a valid decimal digit
+	required qmi_result r = 2a;
+} = 043;
+
+indication test_indication {
+	optional u64 value = 0x99;
+} = 0x7;
diff --git a/tests/bad_hex.qmi b/tests/bad_hex.qmi
new file mode 100644
index 0000000..1e6175c
--- /dev/null
+++ b/tests/bad_hex.qmi
@@ -0,0 +1,19 @@
+package test;
+
+struct qmi_result {
+	u16 result;
+	u16 error;
+};
+
+request test_request {
+	# Note that 'g' is not a valid hexadecimal digit
+	required u8 test_number = 0x1g;
+} = 0x23;
+
+response test_response {
+	required qmi_result r = 2;
+} = 043;
+
+indication test_indication {
+	optional u64 value = 0x99;
+} = 0x7;
diff --git a/tests/bad_octal.qmi b/tests/bad_octal.qmi
new file mode 100644
index 0000000..10c43be
--- /dev/null
+++ b/tests/bad_octal.qmi
@@ -0,0 +1,19 @@
+package test;
+
+struct qmi_result {
+	u16 result;
+	u16 error;
+};
+
+request test_request {
+	# Note that '8' is not a valid octal digit
+	optional test_struct foo = 028;
+} = 0x23;
+
+response test_response {
+	required qmi_result r = 2;
+} = 043;
+
+indication test_indication {
+	optional u64 value = 0x99;
+} = 0x7;